The latest discovery prompted the National History Museum in Denmark to warn: ‘Keep your swimwear on if you’re bathing in the Sound these days – maybe there are more out there!’

The specimen is the first one to have been caught at sea in Europe, the museum added.

‘The pacu is not normally dangerous to people but it has quite a serious bite, there have been incidents in other countries, such as Papua New Guinea where some men have had their testicles bitten off,’ Henrik Carl, a fish expert at the Danish museum, said.

‘They bite because they’re hungry, and testicles sit nicely in their mouth,’ he told English language news website The Local.

The fish was found in the Oresund strait between Denmark and Sweden (Picture: Google Maps)

‘And its mouth is not so big, so of course it normally eats nuts, fruit, and small fish, but human testicles are just a natural target.

‘It’s not normal to get your testicles bitten off, of course, but it can happen, especially now in Sweden.’

The pacu was similar in appearance to its notorious cousin, the piranha, he said.

‘They are almost identical to the piranha, you couldn’t even tell from the outside. It’s just that they have different teeth. Flatter and stronger, perfect for crushing,’ he added.

Mr Carl said that the discovery of one pacu – which is currently undergoing DNA tests to confirm its identity – should not keep Swedish men out of the water.

But if more were found in the area, it could become a serious issue, he suggested.

‘This one was the first, but who knows, it’s probably not the last,’ he added.

In areas where pacus proliferate, fishermen have reportedly bled to death after losing their testicles to the fish’s crushing jaws.